The general purpose of a damping roller within a damping unit of a printing press is to apply a damping medium to some preselected surface, such as a printing plate carried on a rotating plate cylinder. To this end, it is desirable to have a damping roller with an optimum amount of "wettability." A damping roller which is wettable can receive and apply the damping medium. A damping roller with comparatively increased wettability denotes that this particular damping roller can receive comparatively increased quantities of damping medium. On the other hand, a decrease in roller wettability results not only in a decrease in the damping medium application rate but also often results in a non-uniform application of the damping medium. Without an optimum damping medium application rate, the printing process yields ink variations and associated printing problems. It is essential to a successful printing process to maintain a steady damping application rate.
Intimately related to the wettability and the damping medium application rate of a particular damping roller is the damping roller's surface material. A large variety of materials, coverings, or surface structures are known for damping rollers. Damping units traditionally have rollers made of steel, rubber, or textile coverings. Other damping roller surfaces are made of material permeable to a liquid such as porcelain or fired clay (DE-PS 580 963). Prior art damping roller coverings have also been made of elastic yarn (DE-PS 2 607 255) or plastic fibers embedded in resin (EP 0 293 551). Yet another patent (EP 0 400 621) discloses a damping roller whose shell is coated with ceramic/metal carbides. In some cases, these roller surfaces can become "wettable" only if hydrophilic agents, such as gum arabic, are applied to the damping roller surface.
The main disadvantage of these damping rollers is that alcohol or another suitable agent must be added to the damping medium to increase the roller's wettability. If the alcohol is eliminated or if the ratio of the alcohol to the total mixture is reduced to less than 6%, the wettability of these rollers significantly decreases. In contrast, however, damping media that contain less than 6% alcohol in the total damping mixture are considered optimum in the printing business. Hence, a need exists for a damping roller which uses a low damping medium additive concentration while still maintaining a high wettability.